Tubular materials intended to be used as fuse casings, or fuse tubes, must meet a large number of requirements. They must be able to withstand the high pressures generated inside of fuses when blowing, to withstand without ageing significantly the temperatures prevailing in fuses while the latter are carrying current, be heat-shock resistant, be dimensionally stable, substantially non-tracking, cost-effective, etc. Some of the requirements under consideration can be met easily. There are, however, very few tubular materials which meet most or all requirements for a large number of different fuse casing applications, and the cost-effectiveness of these few materials is quite small.
There are fuses whose casings are of ceramic materials and others whose casings are substantially of organic materials, particularly synthetic resins. Synthetic resin casings are generally reinforced by glass fibers in various forms. Among the synthetic resin casings those which are reinforced by convolutely wound glass cloth have probably found the widest acceptance, and electric fuses having filament wound casings are also in use.
The prime object of this invention is to provide fuses, and in particular current-limiting fuses, having casings of glass fiber reinforced synthetic resins that are more cost-effective than prior art fuses of this description.
Another object of this invention is to provide fuses whose casings consist of sections of pultruded tubing. This object is predicated on the fact that the pultrusion process as such is cost-effective, and that it can readily be carried out with synthetic resins that do not evolve gases when heated that are detrimental to arc extinction.
Prior art pultruded tubing lacks the mechanical properties required and/or desirable for casings of electric fuses. It is, therefore, a further object of this invention to provide novel fuse casing structures that lend themselves to be manufactured by the pultrusion process.
Still another object of the invention is to provide fuses, in particular current-limiting fuses, having composite casings particularly designed to maximize the impact strength, or dynamic strength, thereof.
Other objects of this invention will be apparent from what follows.
A current-limiting fuse may be defined as a fuse capable of interrupting the solid metallic current path formed by the fusible element before the current rises to its available peak value, and capable of generating such arc voltages that the current decreases rapidly to zero from the peak value it had reached. This invention refers particularly to current-limiting fuses. By precluding a short-circuit current from reaching the peak value it is capable of reaching, current-limiting fuses greatly reduce stresses of an electromagnetic, or electrodynamic, or thermal nature upon any piece of equipment which is arranged in circuits protected by such fuses. This stress reduction applies not only to miscellaneous electric equipment but also in regard to the stresses to which the casings of current-limiting fuses are subject.
Fusion of the fusible elements of current-limiting fuses and their consequent vaporization is in the nature of an explosion involving generation of relatively high pressures at a relatively high rate. The pressure waves generated at the arc paths in current-limiting fuses are propagated through the pulverulent arc-quenching filler thereof -- which is generally quartz sand -- and act upon the inner side of the walls of the casings of the fuses. Arc-quenching fillers dampen, or attenuate, the intensity of the pressure waves, but their magnitude is still, or may be still, large and tend to cause bursting of casings of fuses. This must, of course, be prevented. It can be prevented by increasing the wall thickness of casings, which may result in intolerable dimensions. This crude approach to the problem of designing fuse casings is also conducive to expensive casing materials.
The forces which mechanical structures can withstand without fracturing depend upon the duration of the action of the forces. Assuming that a given force p maintained during a given period of time T is capable of fracturing a given mechanical structure; that mechanical structure may withstand a much larger force P if applied during a much shorter time t (p &lt; P; t &lt; T). This explains why a piece of electric equipment rated to withstand the forces k exerted by a given short-circuit current i during one second can withstand much higher forces K exerted by a much larger let-through current I of current-limiting fuses whose duration is in the order of milliseconds t(i &lt; I; k &lt; K t &lt; 1 sec.). Thus the fact that the pressure waves of exploding fusible elements acting upon the casings of current-limiting fuses have durations in the order of milliseconds may be utilized to design relatively inexpensive casings for such fuses, i.e. casings which would be mechanically destroyed if the pressure wave were of relatively longer duration, e.g. lasted for 1 second. The maximal force which a given structure may withstand if that force is applied but once and if the duration of that force is very short, e.g. less than 5 milliseconds, may be referred-to as the dynamic strength, or impact strength, of the structure.
The present invention provides novel tubing materials specially evolved to comply with the dynamic or impact strength requirements of casings of current-limiting fuses, and evolved to take advantage of the fact that the casings of fuses are only once in their life, and only for very short durations, subject to very high pressures.
In order to fracture a given structure by an impulse load a predetermined minimum amount of energy is required. This predetermined amount of energy applied by almost instantaneous or shock loading is a measure for the dynamic strength, or impact strength, required for the casings of fuses, in particular current-limiting fuses. Any tubular material intended to be used as casing material for such fuses ought to be constructed to have a high dynamic strength, or impact strength, rather than to be capable of containing high pressures for prolonged periods of time.
Materials which are resilient or ductile tend to be capable of absorbing larger amounts of energy under impact loading conditions than materials which are brittle. Hence a tubular material specifically developed as a fuse casing material is likely to have the required high dynamic strength, or impact strength, if it is resilient, or ductile, rather than brittle.
Synthetic resin casings of electric fuses required to have a large impact strength must include glass fiber reinforcements. These glass fiber reinforcements may take different forms. Presently synthetic resin casings of fuses generally include convolutely wound woven glass fabric, or filament wound glass fibers.
It is a generally accepted view that the difference between theoretical and observed strength of materials, including glass fibers, is caused by structural irregularities. Extremely fine glass fibers freshly drawn from the melt have optimal strength. If such fibers come into contact with a hard object impairing their surface perfection, their strength is greatly reduced. One may conclude from this established fact that it is desirable to minimize any manipulation of glass fibers intended as reinforcement for synthetic resin fuse tubes which tends to impair their surface perfection. The process of weaving glass fibers involves some fiber manipulation. Yet woven glass fiber fabrics have proven so effective in imparting a high impact strength to fuse casings that it appears sensible not to discard this effective reinforcement material.
It has been found that multiply casings including as reinforcement plies of woven fiber glass cloth and at least one ply of non-woven substantially random oriented glass fibers have a significant dynamic or impact strength.
The behavior of any woven fabric, including glass fiber cloth, in response to external forces depends upon the direction of such forces. A given force acting either in the direction of the warp, or in a direction at 90.degree. to it, i.e. the direction of the weft, tends to have a minimal effect on the integrity of the fabric. Forces acting at about 45.degree. to the warp and the weft of a woven material tend to displace some of the fibers relative to the others. Thus, as far as response to external forces is concerned, woven fabrics are strongly polarized in two principal directions, i.e. that of the warp and that of the weft.
The behavior of mats or webs of glass fibers and of non-woven fabrics of glass fibers in response to external forces is entirely different from that of woven fabrics of glass fibers and can be controlled by the orientation of the constituent fibers thereof. This orientation may vary from substantial unidirectionality to substantial randomness. Therefore, the mechanical behavior of a fuse casing can be controlled within limits by providing the casing with plies of woven glass fiber cloth and one or more plies formed by a mat or mats of glass fibers, or a non-woven glass fiber fabric. For reasons which will become more apparent from what follows below the non-woven ply or plies ought to be sandwiched between two plies of woven glass fiber cloth.
In order to form by pultrusion fuse casings having such multiply reinforcements both plies of woven glass fiber cloth ought to have an extent in excess of 360.degree., each forming an overlap extending in a direction longitudinally of the casing. Wherever there is an area of such an overlap, the initial thickness of the woven glass cloth reinforcement insert is doubled. In the pultrusion die the overlapping regions of the plies of woven glass fiber fabric are more compressed than the non-overlapping regions of the woven glass fiber fabric and in the pultrusion die the overlapping highly compressed regions of the plies of glass fiber fabric are radially displaced into the ply of glass fiber mat or non-woven glass fiber fabric. This results in a tubing that has for all practical purposes a uniform wall thickness.
For a better appreciation of the invention its genesis may be compared with that of other fuse casing materials, e.g. that of filament wound fuse casings. Filament winding is a process involving the step of wrapping resin-impregnated continuous filaments of glass around a mandrel in successive layers, thus forming a hollow solid of revolution. This process was not invented for the specific purpose of making casings for fuses, but it was found that filament wound tubing lends itself relatively well as a fuse casing material. Since filament wound tubing was not invented for the specific purpose of forming fuse casings, the performance characteristics inherent in filament wound fuse casings do not perfectly match those required for fuse casings. In order to comply with the specific performance characteristic required for electric fuse casings, filament wound fuse casings must be overdesigned in terms of cost and may, in certain aspects, exceed the requirements actually imposed upon casings of electric fuses.
The same as stated above in regard to filament wound casings of electric fuses is also true in regard to other construction methods of casings of electric fuses.
Casings for electric fuses embodying the present invention are an example for the general rule that components specifically evolved to perform specific functions are likely to have better performance characteristics and/or to be more cost-effective, than components which have not been evolved for a specific purpose.
The casings for fuses embodying this invention may be manufactured at cost of the same order as casings of vulcanized fiber. However, the performance characteristics of the former are far superior. One of the many great advantages of fuses embodying the present invention over fuses having casings of vulcanized fiber lies in the far greater dimensional stability of the glass cloth reinforced casings. Such casings greatly facilitate assembly operations on account of their dimensional stability, and the closeness of the tolerances which they keep. The relative resiliency of the casings of fuses embodying this invention is another significant advantage in regard to the assembly of such fuses. Fitting of the ends of a relatively flexible casing into metallic terminal caps or ferrules is analogous to fitting a resilient stopper of rubber into the rigid neck of a glass bottle. The same analogy applies in regard to fitting rigid metallic terminal plugs into the ends of a relatively flexible casing. Here the rigid metallic terminal plugs take the place of the rigid bottle neck, and the resilient casings take the place of the resilient stoppers of rubber.
The above difference in behavior of casings of fuses embodying this invention and of casings of prior art fuses is not readily apparent to the naked eye but can readily be shown by applying appropriate optical magnification.
It can also be shown that under comparable conditions it requires greater pressure inside prior art fuse casings to blast off caps which are simply mounted on them than to blast off caps from casings of fuses embodying the present invention.